I had the same with someone receiving the pony and then claiming that she paid too much for the shipping/ handling fee and asking for a refund of said fee (and threatening me of a bad review if i didn't complied!!)..
As i refused to yield, she left me a bad review when even Paypal sided with me against her (they usually always side with the buyer).
Oddly enough, even with that clear tentative of extorsion, Etsy refused to remove the review as it "only reflected her view on the transaction, without being insulting"...
So i gather you can slack someone, as long as you stay more or less polite...